Production method for sintered valve guide

ABSTRACT

A production method for a sintered valve guide includes preparing a raw powder which is primarily made of an iron powder and which includes at least a copper alloy powder and a graphite powder. The production method further includes compacting the raw powder into a green compact having an approximately cylindrical shape and sintering the green compact at 950 to 1050° C. The iron powder includes particles, which do not pass through a sieve of 240 mesh, at not less than 40 mass %, and not less than 70 mass % of the powder particles have not more than 0.5 of degree of circularity.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a valve guide that may be used for an intake valve or an exhaust valve in an internal combustion engine. Specifically, the present invention relates to a production method for a sintered valve guide that is produced by a powder metallurgical method and has high strength.

2. Background Art

A valve guide may be press fitted in a cylinder head of an engine and has an inner circumferential surface for slidably maintaining a valve that may be driven so as to take fuel gas into a combustion chamber of an internal combustion engine and to exhaust combustion gas. Therefore, the valve guide is required to have high wear resistance so as to not be greatly worn in sliding on the valve and is also required to have good sliding characteristics so as to not wear the valve of the corresponding sliding member.

Valve guides made of a cast iron are generally used, but valve guides have been produced by powder metallurgical methods recently. This is because in the powder metallurgical methods, alloys having a metallic structure, which cannot be obtained from ingot materials, can be obtained, and therefore characteristics such as wear resistance are improved. Moreover, a member having a shape nearly that of a product is formed, and it is thereby possible to reduce the steps for working and to reduce the amount of material loss. Furthermore, once a die assembly has been made, products having the same shape can be mass-produced. The applicant of the present invention proposed sintered valve guide materials having superior wear resistance, for example, in Japanese Examined Patent Publication No. 55-034858, Japanese Patent No. 2680927, Japanese Patent Application of Laid-Open No. 2002-069597, and Japanese Patent Application of Laid-Open No. 2006-052468.

A sintered valve guide material disclosed in Japanese Examined Patent Publication No. 55-034858 consists of, by mass %, 1.5 to 4% of C, 1 to 5% of Cu, 0.1 to 2% of Sn, not less than 0.1% and less than 0.3% of P, and the balance of Fe. This material has a metallic structure in which a hard iron-phosphorus-carbon compound phase (steadite phase) which is made of eutectic compound of Fe, P and C, a soft Cu—Sn phase (copper alloy phase), and a free graphite phase which functions as a solid lubricant are dispersed in a mixed matrix of pearlite and ferrite. Therefore, a member made of this material has superior wear resistance compared to a member made of a cast iron. In addition, although the member made of this material is not easily cut compared to a member made of a cast iron, the member made of this material has improved machinability compared with that of a member made of a conventional iron-based sintered alloy. For these reasons, this material has been used by various automobile manufacturers.

A sintered valve guide material disclosed in Japanese Patent No. 2680927 is an improved material of the sintered valve guide material disclosed in Japanese Examined Patent Publication No. 55-034858. In this material, machinability is improved while maintaining wear resistance by dispersing magnesium silicate mineral in the above metallic structure. This sintered valve guide material shows superior wear resistance equivalent to the wear resistance of the sintered valve guide material disclosed in Japanese Examined Patent Publication No. 55-034858. The machinability of this material is improved but is still inferior to the machinability of a member made of a cast iron, and therefore, further improvement of the machinability of this material is required. The applicant of the present invention conducted development primarily to improve the machinability even at the cost of slight decrease in wear resistance, and developed sintered valve guide materials disclosed in Japanese Patent Application of Laid-Open No. 2002-069597 and Japanese Patent Application of Laid-Open No. 2006-052468.

In the sintered valve guide materials disclosed in Japanese Patent Application of Laid-Open No. 2002-069597 and Japanese Patent Application of Laid-Open No. 2006-052468, the machinability is improved by decreasing the amount of P in a sintered valve guide material disclosed in Japanese Examined Patent Publication No. 55-034858. The valve guide material disclosed in Japanese Patent Application of Laid-Open No. 2002-069597 consists of, by mass %, 1.5 to 4% of C, 1 to 5% of Cu, 0.1 to 2% of Sn, not less than 0.01% and less than 0.1% of P and the balance of Fe. This material exhibits a structure in which a copper alloy phase and free graphite are dispersed in a matrix primarily made of pearlite. The valve guide material disclosed in Japanese Patent Application of Laid-Open No. 2006-052468 consists of, by mass %, 1.5 to 2.5% of C, 3.5 to 5% of Cu, 0.3 to 0.6% of Sn, 0.04 to 0.15% of P and the balance of Fe. This material exhibits a metallic structure made of a matrix including a pearlite phase, a steadite phase, and a copper alloy phase, pores, and a graphite phase. In this case, the area ratio of the pearlite phase, the copper alloy phase, and the steadite phase, and the thickness of the steadite phase are limited. These sintered valve guide materials disclosed in Japanese Patent Application of Laid-Open No. 2002-069597 and Japanese Patent Application of Laid-Open No. 2006-052468 are superior because of having sufficient wear resistance for practical use and having more improved machinability than that of the material disclosed in Japanese Patent No. 2680927.

The sintered valve guide materials disclosed in Japanese Examined Patent Publication No. 55-034858, Japanese Patent No. 2680927, Japanese Patent Application of Laid-Open No. 2002-069597, and Japanese Patent Application of Laid-Open No. 2006-052468 offer good sliding characteristics due to the copper alloy phase and the free graphite phase. The copper alloy phase and the free graphite phase are formed by adding a copper alloy powder and a graphite powder to a raw powder primarily made of an iron powder and then by sintering at a temperature (950 to 1050° C.) in which the copper alloy powder and the graphite powder are not completely dispersed in an iron matrix.

Recently, in order to decrease the friction in internal combustion engines, instead of a direct driving system for driving of a valve that slides on a valve guide, a roller rocker arm has been used for a valve driving system. Accordingly, the valve guide for slidably maintaining the valve may be applied with a load in a direction intersecting the axis, that is, a bending load. Moreover, in accordance with a trend toward increasing the output from internal combustion engines in recent years, conditions in an internal combustion engine has been severe, whereby a bending load applied to a valve guide is increased.

The sintered valve guide materials disclosed in Japanese Examined Patent Publication No. 55-034858, Japanese Patent No. 2680927, Japanese Patent Application of Laid-Open No. 2002-069597, and Japanese Patent Application of Laid-Open No. 2006-052468 are superior and have high wear resistance and good sliding characteristics. These materials are obtained by sintering at 950 to 1050° C. so as not to completely disperse the copper alloy powder and the graphite powder in the iron matrix, as described above. Therefore, mutual diffusion between iron powder particles is not sufficiently performed and mechanical strength is low in these materials compared with ordinary sintered materials for structural members, which are sintered at 1080 to 1200° C. Consequently, improvement of the mechanical strength is required for coping with the above-described increase in the bending load applied to a sintered valve guide.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a production method for improving mechanical strength of a sintered valve guide including a copper alloy phase and a free graphite phase by sintering at 950 to 1050° C.

The present invention provides a production method for a sintered valve guide in order to solve the above problem, and the production method includes preparing a raw powder which is primarily made of an iron powder and which includes at least a copper alloy powder and a graphite powder. The production method further includes compacting the raw powder into a green compact having an approximately cylindrical shape and sintering the green compact at 950 to 1050° C. The iron powder includes particles, which do not pass through a sieve of 240 mesh, at not less than 40 mass %, and not less than 70 mass % of the powder particles have not more than 0.5 of degree of circularity.

The iron powder used in the production method for the sintered valve guide of the present invention includes more irregular particles than those of an ordinary water-atomized iron powder. Therefore, the iron powder particles tend to be engaged with each other by compacting, and contacting area of the iron powder particles is increased. Accordingly, mutual diffusion between the iron powder particles is facilitated even by sintering at 950 to 1050° C. as usual, whereby a sintered valve guide with higher mechanical strength than that of a conventional valve guide is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a scanning electron microscope (SEM) image showing the appearance of experimental powder particles used in a practical example of the present invention, and FIG. 1B is a SEM image showing the appearance of commercially available water-atomized powder particles.

PREFERRED EMBODIMENT OF THE INVENTION

The inventors of the present invention have conducted intensive research on a method for improving mechanical strength of a sintered valve guide including a copper alloy phase and a free graphite phase by sintering at 950 to 1050° C. As a result, the inventors found the following. If iron powder particles are more engaged in compacting, contacting portions between the powder particles are increased. The contacting portions function as the starting points of neck growth during mutual diffusion of the iron powder particles. Therefore, mutual diffusion between the iron powder particles is increased even when the sintering temperature is 950 to 1050° C., whereby mechanical strength of a sintered compact is increased. Usually, when a raw powder primarily made of an iron powder is compacted, in order to engage the iron powder particles with each other, a water-atomized iron powder is used due to having irregular particles compared with particles of a gas-atomized powder. In this case, only particles having specific sizes have an effect for facilitating engagement of the iron powder particles, and engagement of the iron powder particles occurs more often in compacting if more irregular particles among the particles having specific sizes are used.

That is, in the production method for the sintered valve guide of the present invention, an iron powder is formed so as to include large particles, which do not pass through a sieve of 240 mesh (63 μm), at not less than 40%, preferably, not less than 50%. If the amount of the large particles is less than 40% with respect to the total amount of the iron powder particles, engagement of the iron powder particles occurs less often, and contacting portions between the powder particles as starting points for neck growth are decreased. In this case, the amount of fine powder particles is relatively increased, whereby flowability of a raw powder is decreased, and filling density is decreased because bridging tends to occur.

In addition, the above large particles preferably include rough particles, which have not more than 0.5 of degree of circularity, at not less than 70%. The degree of circularity is a ratio of an actual area of a powder particle to an area of a virtual circle calculated from a perimeter of the powder particle, which may be obtained by observing a SEM image of the powder particle. When a perimeter of a powder particle is represented as L and the area of the powder particle is represented as S, the degree of circularity is represented as 4πS/L² and is in a range of more than 0 and not more than 1 (corresponding to a perfect circle). A powder particle having a degree of circularity nearer 1 has a shape that is near the shape of a perfect circle and is roundish. In contrast, a powder particle having a smaller degree of circularity has a more irregular shape. Such degree of circularity may be calculated by using image analyzing software, such as “WinROOF” produced by Mitani Corporation.

A commonly used water-atomized powder includes large particles of an iron powder, which do not pass through a sieve of 240 mesh, and these large particles include rough particles, which have not more than 0.5 of degree of circularity, at only approximately 50 to 60%. On the other hand, in the present invention, the above large particles of the iron powder include the rough particles with irregular shapes at not less than 70%. Therefore, engagement of the iron powder particles is increased in compacting, and contacting portions between the powder particles as starting points of neck growth are increased, whereby degree of mutual diffusion between the powder particles is increased in sintering. Accordingly, mutual diffusion between the iron powder particles is facilitated even when the iron powder is sintered at 950 to 1050° C. as usual, whereby a sintered valve guide with higher mechanical strength than that of a conventional valve guide is obtained.

In the present invention, an iron powder with high purity is preferably used as the above iron powder. This iron powder of high purity consists of Fe and not more than 0.4 mass % of inevitable impurities, such as C, Si, Mn, P, S, and O, and the amount of O in the inevitable impurities is not more than 0.2 mass %. Since such iron powder of high purity has good compressibility, the powder particles are deformed in compacting, and contacting areas at the above-described contacting portions between the iron powder particles are thereby increased, whereby neck growth is more facilitated in sintering.

The above iron powder including irregular particles may be obtained by the following. For example, fine particles of an atomized iron powder having small amounts of impurities are baked so as to diffusion bond the iron fine powder particles together, and the obtained ingot of the particles are crushed and then particle size is controlled. That is, the atomized fine powder particles having small amounts of impurities are mutually diffusion bonded so that one iron powder particle is formed, whereby the surface area of the iron powder particle is larger than that of a single atomized iron powder particle. The fine particles of the atomized iron powder may be collected by classifying particles of an ordinary atomized iron powder, or may be obtained by atomizing an iron powder so as to form fine particles. If the fine particles of the atomized iron powder are baked in reducing gas, oxides of the powder are reduced during baking, and the amount of the oxides are further decreased.

Ore reduced iron powders are obtained by reducing an iron ore of good quality so as to purify, whereby the ore reduced iron powders are porous and have irregular particles. However, the ore reduced iron powders include impurities more than those of an atomized iron powder and have a large amount of oxides, thereby exhibiting low compressibility. Consequently, deformation amount of the powder particles is small in compacting, and contacting area between the powder particles is small. Accordingly, the ore reduced iron powders are undesirable.

In the present invention, the mechanical strength of a sintered valve guide is improved by using the above-described powder as an iron powder. Therefore, a conventional powder may be used for a raw material such as a copper alloy powder and a graphite powder, except for an iron powder. That is, only an iron powder included in a conventional raw powder is changed for the iron powder of the present invention, and other powders included in the conventional raw powder can be used. By mixing these powders into a raw powder, compacting the raw powder into a green compact, and sintering the green compact at 950 to 1050° C. as usual, a sintered valve guide with improved mechanical strength is obtained. This sintered valve guide exhibits a similar metallic structure to that of a conventional valve guide and has wear resistance and sliding characteristics at the same degree as those of a conventional valve guide.

In this case, by setting the amount of a copper alloy powder in the raw powder so that the amount of Cu is 1 to 5 mass % with respect to the overall composition, an appropriate amount of a copper alloy phase is dispersed in the sintered valve guide. In addition, by setting the amount of a graphite powder in the raw powder so as to be 1.5 to 4 mass %, an appropriate amount of a free graphite phase is dispersed in the sintered valve guide.

An iron-phosphorous alloy powder may be added in the raw powder so that the amount of P will be not less than 0.01 and will be less than 0.1 mass % with respect to the overall composition, whereby a sintered valve guide, in which an iron-phosphorous-carbon compound phase (steadite phase) is dispersed, is obtained. The amount of this iron-phosphorous-carbon compound phase corresponds to the amount disclosed in Japanese Patent Application of Laid-Open No. 2002-069597. By adding an iron-phosphorous alloy powder so that the amount of P will be not less than 0.1 and will be less than 0.3 mass % with respect to the overall composition, a sintered valve guide, in which an iron-phosphorous-carbon compound phase (steadite phase) is dispersed, is obtained. The amount of this iron-phosphorous-carbon compound phase corresponds to the amount disclosed in Japanese Examined Patent Publication No. 55-034858.

Moreover, at least one of a manganese sulfide powder and a magnesium silicate mineral powder may be added to the raw powder at not more than 1.6 mass %. In this case, the machinability of a sintered valve guide is improved as disclosed in Japanese Patent No. 2680927, Japanese Patent Application of Laid-Open No. 2002-069597, and Japanese Patent Application of Laid-Open No. 2006-052468.

EXAMPLES

Fine particles of a water-atomized iron powder were baked so as to diffusion bond the fine iron powder particles together, and the obtained ingot was crushed, whereby an experimental powder was obtained. In addition, a commercially available water-atomized powder was prepared. The experimental powder included large particles which do not pass through a sieve of 240 mesh (63 μm), and these large particles included rough particles, which had not more than 0.5 of degree of circularity, at 80%. The commercially available water-atomized powder also included the large particles, and these large particles included the rough particles at 52%. SEM images of the experimental powder particles and the commercially available water-atomized powder particles are shown in FIGS. 1A and 1B, respectively. The experimental powder particles had more rough surfaces than those of the commercially available water-atomized powder particles. The experimental powder included impurities at 0.29 mass % in which the amount of 0 was 0.07 mass %. The commercially available water-atomized powder included impurities at 3.3 mass % in which the amount of O was 0.12 mass %.

The above large particles of the experimental powder and particles of the experimental powder that passed through the sieve of 240 mesh were mixed by changing the mixing ratio. Moreover, the above experimental powder and the commercially available water-atomized powder were mixed so as to change the ratio of the rough particles, whereby iron powders were prepared. Among the iron powders, the amount of the large particles and the amount of the rough particles varied as shown in Table 1.

Then, 1.8 mass % of a graphite powder, 5 mass % of a copper alloy powder, 0.4 mass % of an iron-phosphorous alloy powder, and 1 mass % of a manganese sulfide powder were mixed with the iron powder into a raw powder. The copper alloy powder consisted of 10 mass % of Sn and the balance substantially of Cu. The iron-phosphorous alloy powder consisted of 20 mass % of P and the balance substantially of Fe. The raw powder was compacted at a compacting pressure of 650 MPa into a green compact having a cylindrical shape with an outer diameter of 18 mm, an inner diameter of 10 mm, and a height of 10 mm. Then, the green compact was sintered at 1000° C. in an ammonia decomposed gas atmosphere, whereby samples of Sample Nos. 01 to 08 were formed. The samples corresponded to a sintered valve guide disclosed in Japanese Patent Application of Laid-Open No. 2006-052468. The samples were subjected to a compressive strength test and compressive strengths were measured. These results are also shown in Table 1. The sample of Sample No. 06 is an example of using only the commercially available water-atomized powder, that is, a conventional example.

TABLE 1 Iron powder Amount of large Amount of Compressive Sample particles rough strength No. % particles % MPa Notes 01 20 80 720 Comparative example 02 30 80 750 Comparative example 03 40 80 850 Practical example 04 40 70 800 Practical example 05 40 60 750 Comparative example 06 40 52 700 Conventional example 07 50 80 850 Practical example 08 60 80 900 Practical example

By comparing the samples of Sample Nos. 01 to 03, 07, and 08 in Table 1, the effect of the ratio of the large particles in the iron powder was investigated. In these samples of Sample Nos. 01 to 03, 07, and 08, the rough particles were included at 80% in the large particles. In the samples of Sample Nos. 01 and 02 including the large particles at less than 40%, the amount of the large particles, which facilitate engagement of the iron powder particles, was small, and the amount of particles passed through a sieve of 240 mesh was excessive. Accordingly, the flowability of the raw powder was decreased, and bridging tended to occur, whereby fillability was decreased. As a result, the samples of Sample Nos. 01 and 02 exhibited low compressive strength. On the other hand, in the samples of Sample Nos. 03, 07, and 08 including the large particles at 40% or more, the amount of the large particles, which facilitate engagement of the iron powder particles, was sufficient. Therefore, the contacting area of the powder particles was increased, and the amount of diffusion due to sintering was increased, whereby the compressive strength was improved.

By comparing the samples of Sample Nos. 03 to 06 in Table 1, which included the large particles at 40 mass %, the effect of the amount of the rough particles was investigated. In the samples of Sample Nos. 05 and 06 including the rough particles at less than 70% in the large particles, the iron powder particles were engaged with each other less often, whereby the compressive strength was low. On the other hand, in the powders of Sample Nos. 03 and 04 including the rough particles at 70% or more in the large particles, the iron powder particles were engaged with each other more often, and the amount of diffusion due to sintering was increased, whereby the compressive strength was improved.

As described above, in the production method for the sintered valve guide, a raw powder, which is primarily made of an iron powder and includes at least a copper alloy powder and a graphite powder, is compacted into a green compact having an approximately cylindrical shape, and the green compact is sintered at 950 to 1050° C. In the production method, when the large particles were included at not less than 40 mass % in the iron powder and the rough particles were included at not less than 70 mass % in the large particles, the mechanical strength of the sintered valve guide was improved. 

1. A production method for a sintered valve guide, comprising: preparing a raw powder primarily made of an iron powder and including at least a copper alloy powder and a graphite powder; compacting the raw powder into a green compact having an approximately cylindrical shape; and sintering the green compact at 950 to 1050° C., wherein the iron powder includes particles, which do not pass through a sieve of 240 mesh, at not less than 40 mass %, and not less than 70 mass % of the powder particles have not more than 0.5 of degree of circularity.
 2. The production method for the sintered valve guide according to claim 1, wherein the iron powder consists of Fe and not more than 0.4 mass % of impurities, and the impurities include not more than 0.2 mass % of
 0. 3. The production method for the sintered valve guide according to claim 1, wherein the copper alloy powder is added to the raw powder so that the amount of Cu is 1 to 5 mass % with respect to the overall composition, and the graphite powder is added to the raw powder at 1.5 to 4 mass %.
 4. The production method for the sintered valve guide according to claim 1, wherein the raw powder includes an iron-phosphorous alloy powder so that the amount of P is 0.01 to 0.3 mass % with respect to the overall composition.
 5. The production method for the sintered valve guide according to claim 1, wherein the raw powder includes at least one of a manganese sulfide powder and a magnesium silicate mineral powder at not more than 1.6 mass %. 